1. Field of the Invention
This invention relates to a device for controlling a thyristor converter used in, for example, a high voltage DC transmission system.
2. Description of the Related Art
Conventionally, as a method for firing thyristors connected in series or in series-parallel in a thyristor converter by a light signal with a narrow pulse width, such method has been used as a gate command is generated when the logic AND condition of a conduction command signal for commanding the conduction duration of the thyristor converter and the forward voltage signal of the thyristors in the thyristor converter is realized.
Hereinafter an operation will be described of a conventional control device for a thyristor converter. FIG. 4 shows a block diagram of a conventional control device for a thyristor converter. In FIG. 4, a plurality of light-triggered thyristors 1-1 through 1-N are connected in series. In parallel to each of the thyristors, a voltage dividing circuit 2 and one of a forward voltage detection circuit 6-1 through 6-N are connected. The circuit 2 consists of a capacitor and resistors. Each of the circuits 6-1 through 6-N consists of a light emitting diode 3, a resistor 4 connected in series to the diode 3 and a diode 5 connected in anti-parallel to the diode 3, respectively. Each of the circuits 6-1 through 6-N detects a forward voltage of one of the thyristors 1-1 through 1-N and generates an optical forward voltage signal, respectively. Each of the forward voltage signals is transmitted to one of light-electricity conversion circuits 8-1 through 8-N via one of light guides 7-1 through 7-N, and is converted to one of electrical forward voltage signals a-1 through a-N therein, respectively. The signals a-1 through a-N are applied to an OR circuit 9 which generates a forward voltage OR signal b.
A conduction command signal c which commands the conduction duration of the thyristor converter and the forward voltage OR signal b are applied to an AND circuit 10 which generates a gate signal d. The gate signal d is applied to a pulse circuit 11 and to one input of an 0R circuit 12. The pulse circuit 11 is a monostable multivibrator which produces a pulse signal with a pulse width T1, for example, 50 microseconds. The output signal of the pulse circuit 11 is applied to the other input of the OR circuit 12. The output signal of the OR circuit 12 is applied to an amplifier circuit 13 for driving series connected light emitting diodes 14-1 through 14-N and 14-M. Each of the light emitting diodes 14-1 through 14-N generates a light gate signal which is supplied to one of the thyristors 1-1 through 1-N, respectively. The light emitting diode 14-M provided for monitoring generates a light gate monitoring signal, which is converted into an electrical gate monitoring signal e in a light-electricity conversion circuit 15. The signal e is applied to a delay circuit 16 which detects whether or not the pulse width of the signal e is smaller than the time duration T2, for example, 1 millisecond.
In the normal operation of the thyristor converter, when the light gate signals are applied to the light-triggered thyristors 1-1 through 1-N, they will be fired and the forward voltage thereof will become to zero. Accordingly, the pulse width of the light gate signals is T1 as determined by the pulse circuit 11.
When one or more of the thyristors 1-1 through 1-N are not turned ON due to the imbalance of the voltage division among the thyristors 1-1 through 1-N, the pulse width of the forward voltage OR signal b is designed to be so large that all of the thyristors 1-1 through 1-N are fired.
In the above-described conventional control device for the thyristor converter, when one of the light-electricity converter units 8-1 through 8-N is in a failure such that the respective one of the forward voltage signals a-1 through a-N is generated continuously, the gate signal d is generated during the conduction command duration. Then the light gate pulse is generated during that time. When the time T2 lapses since the light gate pulse rises, it is designed that the output of the delay circuit 16 will be "1" thereby to stop the operation of the thyristor converter. The state that the light gate pulse is generated continuously is very dangerous for the thyristor converter, because the light emitting diodes 14-1 through 14-N are not designed to withstand continuous use, so that the operation thereof is to be stopped for repairing the failure portion.
Accordingly, the conventional control device has such a defect that if only one of the photoelectric conversion circuits 8-1 through 8-N for forward voltage detection (the number of the circuits is the same to that of the thyristors) is in a failure that the forward voltage signal is generated continuously, the light gate pulse is generated abnormally and the operation of the thyristor converter is to be stopped.